For example, in an apparatus for driving an in-vehicle motor, to secure a protection in cases where overvoltage occurs owing to load dumping, a measure may be taken which inserts a Zener diode in a power source line to thereby clamp the overvoltage. Power Zener diodes are comparatively expensive. Further, if a Zener diode is selected to have high withstand voltage as a measure against the overvoltage, it becomes more expensive. This poses a problem of increasing the product price. Patent Document 1 describes a technology in which a motor is rotated on maximum output conditions to thereby consume electric power upon detecting an overvoltage applied.
There is a so-called (position) sensorless drive system that estimates a position of a rotor by detecting a phase voltage generated on the motor, without using a position detection element such as a Hall sensor in driving a brushless DC motor. If the above mentioned technology in Patent Document 1 is applied to an apparatus using such a sensorless drive system, the following problem may occur. For instance, if a control described in the above mentioned technology is applied during performing a drive control using the sensorless drive system, position detection signals acquired based on induced voltage fluctuate to thereby fluctuate timing for applying electric current to the motor. As a result, applying electric current may be concentrated on a specific phase or irregular in applying order so that electric power cannot be consumed. Thus, overvoltage protection may not be appropriately performed.                Patent Document 1: JP 2000-69786 A        